minimum sample delay

[KingJL]

How do you determine when coil current has reduced to 1mA?

in anything I build, I always include a 0.1 ohm resistor on the low side of the coil circuit that is accessible for measurement across it. Once you can determine that point, you can make decisions about the excitation and detection of the target that will minimize the things that detract from detection. A couple of things to consider (take away all the math and use a simplified explanation): (1) eddy currents are produced in one direction when the the magnetic field is expanding and the opposite direction when the magnetic field is collapsing (eddy currents subside in a static magnetic field), (2) induced eddy currents, in effect, have an inertia due to and somewhat proportional to the inductive qualities of the target. This "inertia" manifests itself as the time constant of the target. Having "inertia", the eddy currents lag the excitation force of the changing magnetic field... the amount of lag again manifests itself as the time constant. (3) The more (up to a point) you can separate the increasing field from the collapsing field (in other word separate these by a period of static or near-static field strength), you can maximize the target signal that is left for detection by minimizing the cancellation effect of the induced eddy currents (the eddy currents subside between the state change). Toying around with the 3 states (increasing field, constant field, collapsing field, the rate of change, and the magnitude of change), you can come up with the optimum coil current waveform envelope to achieve maximum detection of your desired target. Then you can determine the optimum time within practical limits to sample the received target response.

[green]

One time where it might make sense to trigger on Tx off. Ground should chart a straight line on a log log chart. Does if trigger is Tx off, doesn't if zero time is after Tx off.

Toying around with the 3 states (increasing field, constant field, collapsing field, the rate of change, and the magnitude of change), you can come up with the optimum coil current waveform envelope to achieve maximum detection of your desired target. Then you can determine the optimum time within practical limits to sample the received target response. I have found constant rate vs constant current to give a higher signal if average coil current and Tx time is the same. Have you found different?

[green]

in anything I build, I always include a 0.1 ohm resistor on the low side of the coil circuit that is accessible for measurement across it. Once you can determine that point, you can make decisions about the excitation and detection of the target that will minimize the things that detract from detection. A couple of things to consider (take away all the math and use a simplified explanation): (1) eddy currents are produced in one direction when the the magnetic field is expanding and the opposite direction when the magnetic field is collapsing (eddy currents subside in a static magnetic field), (2) induced eddy currents, in effect, have an inertia due to and somewhat proportional to the inductive qualities of the target. This "inertia" manifests itself as the time constant of the target. Having "inertia", the eddy currents lag the excitation force of the changing magnetic field... the amount of lag again manifests itself as the time constant. (3) The more (up to a point) you can separate the increasing field from the collapsing field (in other word separate these by a period of static or near-static field strength), you can maximize the target signal that is left for detection by minimizing the cancellation effect of the induced eddy currents (the eddy currents subside between the state change). Toying around with the 3 states (increasing field, constant field, collapsing field, the rate of change, and the magnitude of change), you can come up with the optimum coil current waveform envelope to achieve maximum detection of your desired target. Then you can determine the optimum time within practical limits to sample the received target response.

I've been thinking we are discussing what we call zero time when we compare or present data. Do you sample your data with an operating PI based on coil current reaching 1ma instead of Tx off. I would have thought that wouldn't work,
if that is what you are doing and it works better I will give it a try.

[green]

Tried am Impulse simulation with spice. Not the same but maybe close enough. With R3 1ohm(TC=1usec) Impulse has less signal. With R3 .005 ohms(TC=200usec) Impulse has more signal.

[KingJL]

Tried am Impulse simulation with spice. Not the same but maybe close enough. With R3 1ohm(TC=1usec) Impulse has less signal. With R3 .005 ohms(TC=200usec) Impulse has more signal.

You are not even close with the impulse simulation. It's design relies heavily of the coil discharge energy being routed to some serious capacitance clamped to the input supply voltages. Here are simulation files for the basic TX concept. This one is using transistors that are available in LtSpice. I had models for the actual transistors used in the Impulse, but they are on a dead computer. You can play with the large capacitance and the coil values to change the coil current waveform anywhere from the triangular to a half sine. You can vary the coil however you like and adjust the supply voltage to get the desired coil current for test. If you over damp you will destroy short TC response. Base your total damp on the coil capacitance using the function R= SQRT(L/C)/2.

The impulse was introduced here, not because it provided the best current waveform for fast TC detection (I think the limit for it is probably about a 3 usec target) , but to show that the reference to TX switch-off was not really a valid measure to judge sampling. In actuality the best current waveform for your fast TC test is a trapezoidal current waveform with the rise time, the duration of the steady current, and discharge time tailored to your desired TC of detection. DaveJ, I believe, has designed a bi-polar TX that has roughly the following characteristics: fast ramp up (~5 usec) of current to about 1A (roughly 1/4 sine shape), near steady current for 40 usec, 5 usec coil disicharge (roughly 1/4 sine shape). If you take steps to insure all residual coil current is stopped at the zero point during discharge, you could theoretically sample at that time... but in the real world, theory does not equal reality. So you try to achieve as close as possible.

[green]

One time where it might make sense to trigger on Tx off. Ground should chart a straight line on a log log chart. Does if trigger is Tx off, doesn't if zero time is after Tx off.

Toying around with the 3 states (increasing field, constant field, collapsing field, the rate of change, and the magnitude of change), you can come up with the optimum coil current waveform envelope to achieve maximum detection of your desired target. Then you can determine the optimum time within practical limits to sample the received target response. I have found constant rate vs constant current to give a higher signal if average coil current and Tx time is the same. Have you found different?

My error. Tx off probably wouldn't the place for zero time with the Impulse.

maybe not the best for any PI when charting data

[green]

You are not even close with the impulse simulation. It's design relies heavily of the coil discharge energy being routed to some serious capacitance clamped to the input supply voltages. Here are simulation files for the basic TX concept. This one is using transistors that are available in LtSpice. I had models for the actual transistors used in the Impulse, but they are on a dead computer. You can play with the large capacitance and the coil values to change the coil current waveform anywhere from the triangular to a half sine. You can vary the coil however you like and adjust the supply voltage to get the desired coil current for test. If you over damp you will destroy short TC response. Base your total damp on the coil capacitance using the function R= SQRT(L/C)/2.

The impulse was introduced here, not because it provided the best current waveform for fast TC detection (I think the limit for it is probably about a 3 usec target) , but to show that the reference to TX switch-off was not really a valid measure to judge sampling. In actuality the best current waveform for your fast TC test is a trapezoidal current waveform with the rise time, the duration of the steady current, and discharge time tailored to your desired TC of detection. DaveJ, I believe, has designed a bi-polar TX that has roughly the following characteristics: fast ramp up (~5 usec) of current to about 1A (roughly 1/4 sine shape), near steady current for 40 usec, 5 usec coil disicharge (roughly 1/4 sine shape). If you take steps to insure all residual coil current is stopped at the zero point during discharge, you could theoretically sample at that time... but in the real world, theory does not equal reality. So you try to achieve as close as possible.

I think the discharge current is going thru the zener diodes like in my simulation, maybe I'm wrong.

[KingJL]

I think the discharge current is going thru the zener diodes like in my simulation...

Not really! Current rate and shape is affected greatly by the value of the capacitances (and inductance)... not sure that I can help your understanding of the concept. You can only gain that understanding when you have analyzed it's performance with varying values of L and C for tens of hours... then a light switch goes on and you say... "OH I SEE"!

[green]

Not really! Current rate and shape is affected greatly by the value of the capacitances (and inductance)... not sure that I can help your understanding of the concept. You can only gain that understanding when you have analyzed it's performance with varying values of L and C for tens of hours... then a light switch goes on and you say... "OH I SEE"!

Try clicking on L1, D1 and D2 to display current.

[Carl-NC]

The zeners aren't zeners, they're Schottkys. The Schottkys short the flyback to the rails and recharge the tank caps, thereby recycling most of the TX energy. You can run the sim without the tank caps and it will work the same as long as the supply and coil return are ideal batteries. You can replace the BJT switches with MOSFETs, then you can use the body diodes and eliminate the Schottkys. If you reduce the tank caps, you can tune the circuit and make a half-sine transmitter. You can then chop the trailing edge and make a truncated half-sine. You can also full-bridge the thing and double the TX current "for free." There is much fun to be had with this circuit.

[6666]

Hi Carl in your experiments when you use full-bridge, where do you take the target signal from ? thanks.

[Carl-NC]

In most designs I'm using a separate RX coil. In the Fisher PP I use a differential preamp and pick off both sides of the TX.

[6666]

In most designs I'm using a separate RX coil. In the Fisher PP I use a differential preamp and pick off both sides of the TX.

Thanks for reply

[green]

The zeners aren't zeners, they're Schottkys. The Schottkys short the flyback to the rails and recharge the tank caps, thereby recycling most of the TX energy. You can run the sim without the tank caps and it will work the same as long as the supply and coil return are ideal batteries. You can replace the BJT switches with MOSFETs, then you can use the body diodes and eliminate the Schottkys. If you reduce the tank caps, you can tune the circuit and make a half-sine transmitter. You can then chop the trailing edge and make a truncated half-sine. You can also full-bridge the thing and double the TX current "for free." There is much fun to be had with this circuit.

Thanks for the reply, think I'm starting to figure it out. Still working on a coil for 1usec delay. Attempted to chart signal decay with fast decay and decay I think is similar to the impulse. Wondering if it makes sense. Target distance was adjusted to give a near full scale signal for fast decay with coins and foil(couldn't get full scale with ground). I'll have to try a full bridge circuit.

Thanks KingJL for posting it.

[KingJL]

Thanks for the reply, think I'm starting to figure it out. Still working on a coil for 1usec delay. Attempted to chart signal decay with fast decay and decay I think is similar to the impulse. Wondering if it makes sense. Target distance was adjusted to give a near full scale signal for fast decay with coins and foil(couldn't get full scale with ground). I'll have to try a full bridge circuit.

Thanks KingJL for posting it.

As Carl has said... "There is much fun to be had with this circuit".